Hydrogen peroxide (H2O2) is a versatile commodity chemical with diverse applications. Hydrogen peroxide applications take advantage of its strong oxidizing agent properties and include pulp/paper bleaching, waste water treatment, chemical synthesis, textile bleaching, metals processing, microelectronics production, food packaging, health care and cosmetics applications. The annual U.S. production of H2O2 is 1.7 billion pounds, which represents roughly 30% of the total world output of 5.9 billion pounds per year. The worldwide market for hydrogen peroxide is expected to grow steadily at about 3% annually.
Various chemical processes may be employed to manufacture hydrogen peroxide on a commercial scale. One major class of hydrogen peroxide manufacture comprises the auto-oxidation (AO) of a “working compound” or “working reactant” or “reactive compound”, to yield hydrogen peroxide. Commercial AO manufacture of hydrogen peroxide has utilized working compounds in both cyclic and non-cyclic processes.
The cyclic AO processes typically involve hydrogenation (reduction) of a working compound and then auto-oxidation of the hydrogenated working compound to produce hydrogen peroxide. Most current large-scale hydrogen peroxide manufacturing processes are based on an anthraquinone AO process, in which hydrogen peroxide is formed by a cyclic reduction and subsequent auto-oxidation of anthraquinone derivatives. The anthraquinone auto-oxidation process for the manufacture of hydrogen peroxide is well known, being disclosed in the 1930s by Riedl and Pfleiderer, e.g., in U.S. Pat. No. 2,158,525 and No. 2,215,883, and is described in the Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed., Volume 13, Wiley, N.Y., 1981, pp. 15-22.
In addition to the anthraquinones, examples of other working compounds feasible for use in the cyclic auto-oxidation manufacture of hydrogen peroxide include azobenzene and phenazine; see, e.g., Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed., Volume 13, Wiley, N.Y., 1981, pp. 15 & 22.
In commercial AO hydrogen peroxide processes, the anthraquinone derivatives (i.e., the working compounds) are usually alkyl anthraquinones and/or alkyl tetrahydroanthraquinones, and these are used as the working compound(s) in a solvent-containing working solution. The anthraquinone derivatives are dissolved in an inert solvent system. This mixture of working compounds and solvent(s) is called the working solution and is the cycling fluid of the AO process. The solvent components are normally selected based on their ability to dissolve anthraquinones and anthrahydroquinones, but other important solvent criteria are low vapor pressure, relatively high flash point, low water solubility and favorable water extraction characteristics.
Non-cyclic AO hydrogen peroxide processes typically involve the auto-oxidation of a working compound, without an initial reduction of hydrogenation step, as in the auto-oxidation of isopropanol or other primary or secondary alcohol to an aldehyde or ketone, to yield hydrogen peroxide.
Hydrogenation (reduction) of the anthraquinone-containing working solution is carried out by contact of the latter with a hydrogen-containing gas in the presence of a catalyst in a large scale reactor at suitable conditions of temperature and pressure to produce anthrahydroquinones.
Once the hydrogenation reaction has reached the desired degree of completion, the hydrogenated working solution is removed from the hydrogenation reactor and is then oxidized by contact with an oxygen-containing gas, usually air. The oxidation step converts the anthrahydroquinones back to anthraquinones and simultaneously forms H2O2 which normally remains dissolved in the working solution.
The remaining steps in conventional auto-oxidation processes are physical operations. The H2O2 produced in the working solution during the oxidation step is separated from the working solution in a water extraction step. The H2O2 leaving the extraction step is typically purified and concentrated.
The working solution from which H2O2 has been extracted is returned to the reduction step. Thus, the hydrogenation-oxidation-extraction cycle is carried out in a continuous loop, i.e., as a cyclic operation.
An overview of the anthraquinone AO process for the production of hydrogen peroxide is given in Ullman's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A13, pages 447-456.
The oxidation step is critical to the economics of the auto-oxidation hydrogen peroxide process. Typical oxidizer vessels contain a large inventory of working solution. This costly inventory of working solution represents a high working capital cost in addition to the capital cost of such large vessels. Furthermore, the large working solution inventory in conventional oxidizer vessels inherently presents a higher fire risk in the operating plant.
Another drawback of the large reactor inventory is its high reactor residence time, which can lead to unwanted by product formation, such as epoxides, and can result in higher decomposition rates of hydrogen peroxide. Conversion of such byproducts back into useful reactants capable of producing hydrogen peroxide necessitates costly side stream unit operations. Of course, the rate of generation of these parasitic byproducts has a strong influence on the sustained capacity and operating costs of a conventional AO process.
One technical objective of anthraquinone based oxidizers is to maximize reactor volumetric productivity. Another objective is to maximize oxidation reaction conversion of the anthrahydroquinone to produce the desired product, hydrogen peroxide. In addition, formation of byproduct compounds such as epoxides should be minimized. The typical oxidation step requires more electricity (when employing typical motor-driven compression for the oxidizing agent stream) than any other unit operation in the anthraquinone hydrogen peroxide process, so minimization of energy requirements is another objective.
Commercial oxidation reactions are most often carried out in a bubble column or modified bubble column reactor. These column reactors may employ some type of internal packing or plates or, alternatively, may be empty. Bubble column reactors achieve good gas/liquid mixing and maintain a well-mixed liquid phase. Modified bubble reactors are those bubble column reactors employing internal devices such as packing or static mixers or, alternatively, employing flow arrangements that deviate from a single liquid and single gas bottom inlet with corresponding co-current upward flow. Pure oxygen, enriched oxygen, and preferably air are used as the oxidizing agent.
A counter-current anthraquinone oxidation column is described in U.S. Pat. No. 2,902,347, which calls for packed columns to minimize the working solution residence time, to minimize and possibly avoid by-product formation. This column type suffers from the disadvantage of requiring a plurality of series-connected columns to achieve reasonably complete conversion, since oxidizing gas feed rate must be kept relatively low to prevent flooding.
A combination of co-current and counter-current oxidation column configurations, to provide improved mixing, is disclosed in U.S. Pat. No. 3,880,596. The oxidation column of the '596 patent, also described in Chem. Process Eng., Vol. 40 (1959), No. 1, p. 5, utilizes internals such as baffles or packing inside the column, to route working solution and oxidizing gas co-currently through individual sections but counter-currently overall. Drawbacks of these columns are the considerable pressure drop associated with the internal devices and the internal reactor space taken up by these internal devices, requiring large reactor vessels to effect substantially complete oxidation conversion. The reported volumetric productivity for the column oxidation reactor describe in the '596 patent is 15.0 kg H2O2/m3-hr, using air as the oxidizing agent.
EP-A-221 931 describes an oxidation step in a column with no internal fittings, to avoid the column pressure drops associated with the above-noted columns. The oxidizing gas and working solution are mixed in a nozzle before introduction into the empty column. The gas-liquid mixture produces a coalescence-inhibited stable dispersion in which the gas bubbles retain their initial size despite the absence of internal flow devices. A disadvantage of this oxidation reactor design is that the aerated volume is quite large, reducing the volumetric productivity. The reported volumetric productivity is 22 kg H2O2/m3-hr using air as the oxidizing agent.
To improve volumetric reactor productivity over that of the oxidizer reactors mentioned above, U.S. Pat. No. 5,196,179 discloses a tubular reactor with inserted static mixing devices. The working solution and an oxygen-containing mixture flow co-currently in a homogeneous dispersion. The reported volumetric productivity is 254 kg H2O2/m3-hr when utilizing pure oxygen gas.
U.S. Pat. No. 6,375,921 describes a counter-current bubble column oxidizer that employs perforated trays. The trays preferably contain complex tapered holes with a round, triangular, semi-elliptical, or slit-shaped construction. The reported volumetric productivity for the '921 patent column oxidizer is 36.0 kg H2O2/m3-hr using air as the oxidizing agent.
External mixing devises such as venturi nozzles can be employed to enhance the gas-liquid mixing. U.S. Pat. No. 6,426,057 discloses an approach in which a split stream of preoxidized working solution and hydrogenated working solution are mixed with the oxidizing gas in a venturi nozzle before introduction into an empty bubble column reactor.
Oxidation catalysts such as secondary amines (U.S. Published Patent Application No. 2002/0141935) can be added to conventional anthraquinone oxidizers to accelerate the rate of oxidation. The patent application discloses oxidation reactions in a laboratory reactor that proceed 3.4, 14.2, and 9.0 times faster (than a catalyst-free system using air) when adding 1000 ppm di-n-octylamine, 10% di-n-octylamine, and 1000 ppm di-n-butylamine, respectively.
Conventional AO process oxidation reactors have two significant disadvantages-large reactor volumes and correspondingly long liquid residence times. These characteristics lead to additional disadvantages: large equipment, high equipment and construction costs, large plant footprint, high working capital costs, large fire loads associated with large reactor volumes, and unwanted by-product formation and hydrogen peroxide decomposition associated with the longer liquid residence times.
A very different approach for avoiding some of the drawbacks associated with hydrogenation and oxidation reactions in conventional AO processes is the direct synthesis of hydrogen peroxide from reaction of hydrogen and oxygen, which eliminates the separate hydrogenation and oxidation steps. One such direct synthesis process is described in U.S. Pat. No. 7,029,647, in which the staged reaction of the hydrogen and oxygen reactants is carried out in a microchannel reactor.
It is a principal object of this invention to provide an improved process for the oxidation stage of a conventional AO process for producing hydrogen peroxide with high volumetric reactor productivity, minimum by-product formation and hydrogen peroxide loss through decomposition, high oxidizing gas utilization, and high-grade waste heat of reaction. Another object of this invention is to employ reduced reactant inventories over the conventional AO oxidation reactors.
The present invention achieves these and other objectives in the auto-oxidation production of hydrogen peroxide, using an oxidation stage carried out in a microreactor.